The need for AFC in coherently detecting phase modulated signals arises since even small frequency offsets between the transmitter and the receiver reference frequencies can result in a significant number of detected data errors. To demonstrate this problem, consider the following example. Assume data is sent at a 300 Kb/s data rate using Minimum Shift Keying (MSK) (or a variation of this modulation format, such as Gaussian Minimum Shift Keying-GMSK; Generalized Tamed FM-GTFM; etc.) in a Time Division Multiple Access system employing time slots of 0.5 msec in duration. Hence a time slot consists of (300 Kb/s).times.(0.5 msec)=150 bits. Assume further that the phase offset between the transmitter and receiver is adjusted to zero at the start of each received time slot through the use of a synchronization preamble, etc. For noise-free conditions, it can be shown that for an MSK modulation format, bits may be detected without error in the receiver provided that the phase offset between the transmitted and receiver is less than .pi./2 radians. As instantaneous frequency is the time derivative of phase, in order for the time slot to be received without error, it is necessary that the phase offset at the end of the slot be less than .pi./2 radians, i.e., that the frequency offset between the transmitter and the receiver satisfy ##EQU1## To accommodate the effects of noise, in practice, it is necessary that the frequency offset be somewhat smaller than this, typically 200 Hz.
In a mobile radio operating at 900 MHz, a 200 Hz maximum frequency offset between the transmitted carrier and receiver's reference frequency implies that both the transmitter and receiver must employ oscillators having an overall stability (over time, temperature, etc.) of better than 0.1 parts per million (ppm), a stability requirement currently met only by cesium or rubidium frequency standards and ovenized crystal oscillators. All of these oscillators are too bulky for commercial mobile radio applications. Instead, AFC must be employed with a smaller oscillator, compromising frequency stability. Methods must be devised for controlling frequency stability in other ways. AFC circuits are one common way.
Conventional AFC circuits, such as described in J. C. Samuels' "Theory of the Band-Centering AFC System", IRE Transactions on Circuit Theory, pp. 324-330, December 1957 are designed to compensate for large frequency offsets between the transmitter and receiver in order to keep the received signal within the bandwidth of the receiver's IF filter. This is usually accomplished via a frequency discriminator detector whose output is low-pass filtered to remove any data artifacts from the received signal's mean frequency. Such an approach is useful in achieving frequency offsets of approximately .+-.1 KHz at 900 MHz. It is not an acceptable approach towards achieving a frequency offset of less than 200 Hz unless the transmitted signal bandwidth is less than 200 Hz (e.g., a sinusoid). However, even to achieve frequency lock to a sinusoidal signal, it is necessary to carefully further process the received signal.
In practice, in order to achieve initial frequency lock to a received GMSK signal, a sinusoidal burst is transmitted at periodic intervals between data bursts by transmission of a long burst of logic 0's or 1's using the GMSK modulation format. As is well-known, the signal resulting from such a data input is a sinusoidal signal at a frequency equal to the carrier frequency plus or minus one-quarter of the bit rate 1/4T, , i.e. ##EQU2## where the choice of the sign depends upon whether all ones or all zeros are transmitted.
With this sinusoidal burst transmission, locking to the received carrier requires both detection of the sinusoidal burst and frequency locking to the sinusoidal burst.
This invention then takes as its object to overcome these challenges and to realize certain advantages presented below.